In man, pigmentation results from the synthesis and distribution of melanic pigments most notably in the skin, the hair and the pigmentary epithelium of the iris. Thus, the color of the skin, the hair and the eyes depends principally on the types of pigments present and their concentrations. This pigmentation is regulated by many internal or external factors, such as, for example, exposure to ultraviolet radiation.
Melanins are macromolecules produced by melanocytes by the addition or condensation of monomers formed from tyrosine (eumelanin) or of tyrosine and cysteine (pheomelanin) The mechanism by which melanins are synthesized, or melanogenesis, is particularly complex and involves various enzymes, principally tyrosinase and the tyrosine related protein (tyrosinase-related protein-1 or Tyrp-1). During melanogenesis, these enzymes catalyze in particular the conversion of tyrosine into DOPA (dihydroxyphenylalanine) and then into dopaquinone. From this molecule, two metabolic pathways allow the synthesis of either eumelanin or pheomelanin.
Melanogenesis takes place in specialized intracellular organelles contained in melanocytes: melanosomes. Although melanosomes were among the first cellular organelles to be described morphologically (Seiji et al., 1963), their protein composition and their biogenesis remain relatively little-known.
Melanosome maturation can be broken down into four stages on the basis of morphological criteria. Stage I corresponds to a compartment delimited by a membrane with a variable quantity of intraluminal membranes. Stage II is an ellipsoidal structure with characteristic protein-like striations. These striations play an important role in melanin concentration, in the elimination of toxic synthesis intermediates and in facilitating the transfer of melanin to keratinocytes (Seiji et al., 1963). Melanin is detected from stage III by electron-dense deposits along the striations. Stage IV is an electron-dense structure in which the internal striations are no longer visible. This stage corresponds to the mature melanosome ready to be transferred to the keratinocytes (Van Den Bossche et al., 2006).
During the process of biogenesis, the “pigmented” mature melanosome (stages III and IV) is obtained by the addition of enzymes that are key to melanin synthesis and of effectors necessary to its transport toward the periphery (Raposo et al., 2001). The enzymes involved in melanogenesis are thus synthesized in the Golgi apparatus and then transferred in the pre-melanosomes. The mechanisms involved in this transfer are still relatively little-known. This type of transfer requires in particular the participation of adaptor protein (AP) complexes which are heterotetramers having the role of recruiting enzymes in the transport vesicles. There are four of these complexes, named AP-1 to AP-4. The inventors previously showed that AP-3 complex was involved in the transfer of tyrosinase toward melanosomes. However, a lack of AP-3 does not eliminate pigmentation and does not affect the transport of Tyrp-1. They also observed that AP-1 adaptor complex was able to interact with amino acid sequences of cytosolic domains of tyrosinase and Tyrp-1 without being able to elucidate the exact function of this complex in melanogenesis (Theos et al., 2005). Parallel to that, it was also shown that a kinesin, named Kif13A, was able to interact with a sub-unit of AP-1 (Nakagawa et al., 2000). This interaction was observed in cell lines lacking melanosomes. The existence of such an interaction in melanocyte cell lines, as well as its role in melanogenesis, has thus not been established.
Melanocyte dysfunction can lead to pigmentation anomalies. Hypopigmentation can result from depigmentation diseases, in particular albinism and vitiligo. Conversely, local hyperpigmentations can result from certain melanocyte conditions such as idiopathic melasma or benign melanocyte hyperactivity and proliferation causing, for example, senile pigmentary spots (senile lentigo). Hyperpigmentations can also be caused due to an accident, for example, by photosensitization or post-lesion scarring.
These hyperpigmentations can be treated by means of depigmentation substances administered by topical route. A depigmentation molecule acts on skin melanocytes and interferes with one or more stages of melanogenesis. Known depigmentation substances are in particular hydroquinone and its derivatives, ascorbic acid and its derivatives, placental extracts, kojic acid, arbutin, iminophenols (WO 99/22707), the combination of carnitine and quinone (DE 19806947), amino-phenol amide derivatives (FR 2772607), and benzothiazole derivatives (WO 99/24035).
These substances can present various disadvantages due in particular to their instability, the need for using them at high concentrations, their nonspecific action or their toxic, irritating or allergenic properties. Thus, effective and inoffensive depigmentation substances to treat or prevent hyperpigmentation by topical route are highly sought after in the fields of cosmetology and dermatology.
In the last few years, new approaches to treat diseases caused by melanocyte dysfunction have appeared. The use of antisense oligonucleotides was envisaged in particular. Indeed, document WO 99/25819 described oligonucleotides used to increase pigmentation by regulating the expression of tenascin to treat vitiligo and other depigmentation diseases and document FR 2804960 described antisense oligonucleotides to regulate the expression of tyrosinase or Tyrp-1 to treat hyperpigmentations.